1. Field of the Invention
This invention relates to a neutron generating system. More specifically, the invention relates to a new and improved neutron generator tube especially adapted to traverse the narrow confines of a well or borehole for well logging purposes.
2. Description of the Prior Art
Over four decades have passed since F. M. Penning disclosed a neutron generator in U. S. Pat. No. 2,211,668 constructed of a low pressure deuterium-filled envelope containing a cathode and anode with an axially oriented magnetic field ion source, a nuclear reaction producing target and one or more acceleration electrodes. For the last three decades this "Penning" ion source has been employed extensively in various neutron generator tubes for downhole oil and gas well neutron logging. During this period extensive modifications and improvements have been suggested with varying degrees of commercial success, yet specific problems still remain, particularly during well logging in deep wells at high temperatures.
It is generally known that the permanent magnetic materials used in the conventional neutron generator tubes tend to lose their magnetic properties when subjected to temperatures such as 400.degree. C. or greater (see U.S. Pat. Nos. 3,546,512 and 3,756,682). Because of the small confines of a well borehole, the neutron generator tube must have extremely high magnetic field capabilities in a relatively high vacuum in order to have significant ion production. In order to prevent or eliminate outgassing within the tube during use in deep, high temperature wells, an ultra high temperature bake out is necessary during fabrication of the tube. This creates the pragmatic dilemma; i.e., if an external magnet or field is employed (separate from the neutron generator tube) the physical dimensions of the resulting well logging tool restricts its utility, and if an internal permanent magnet is employed, the bake out procedure again will either restrict the physical size or deleteriously affect the magnetic field strength.
Another historically recognized problem which continues to pragmatically limit the contemporary neutron generator tube is the removal of thermal energy from the target surface of the tube. Thus it is known that the energy of the ion beam striking the target and inducing the desired nuclear reaction, if too intense, will result in high temperature sputtering and thermal failure of the target and thus failure of the neutron generator tube. Various methods of modifying the composition and the thickness of the hydrogen occluding target film have been proposed to compensate for this problem. In a recent U.S. Pat. No. 3,784,824, vapor deposition or sputtering of a non-occluder for hydrogen onto the target during operation was suggested; yet, the problem essentially remains as a critical limitation. It is also generally recognized that shielding or confining the magnetic field to the ion source by encapsulating the permanent magnet and ion source and allowing the ion to escape through an aperture creating a narrow intense ion beam (see U.S. Pat. No. 3,112,401) is a desirable practice. In contrast, U.S. Pat. Nos. 3,141,975 and 3,401,264 employ one or more ion beam grids (with and without variable potential) placed between the ion source and target. In this approach the ion beam optics are manipulated across a relatively large cross-sectional ion beam such that the ion path is completely defocused and linear, thus allowing for a low energy acceleration of the positive ions.
However, up to this time the combination of optimum thermal energy removal from the target surface and uniform distribution of the ion beam energy impinging on the target per unit surface area has been beyond contemporary technology.